The present invention relates generally to optical systems, and more particularly to an optical unit that scans a beam radiated from a light source in a predetermined direction. The present invention lends itself, for example, to a scanning optical system for an electrophotographic recording device, and optical disk unit.
The xe2x80x9celectrophotographic recording devicexe2x80x9d by which we mean is a recording device employing the Carlson process as described in U.S. Pat. No. 2,297,691, and denotes a nonimpact image-forming device that provides a recording by depositing a developing agent as a recording material on a recordable medium (e.g., printing paper, and OHP film). The electrophotographic recording device is typified by a laser printer, and is broadly applicable not only to a discrete printer, but also generally to an apparatus having a printing function such as a photocopier and a facsimile unit. The scanning optical system is typically a unit that includes a variety of light sources (e.g., a semiconductor laser, a Hexe2x80x94Ne gas laser, a Ar gas laser, and a Hexe2x80x94Cd gas laser), a collimating lens, a rotating mirror, an f-xc3xa8 lens, a cylinder lens, etc., serving to form a desired latent image on a photosensitive body.
A laser printer as an example of the electrophotographic recording device has characteristics such as an excellent operability and cost efficiency, and high-quality image formation. In addition, due to a reduced vibration and noise during printing, the use of the laser printer for computer""s output devices, facsimile units, photocopiers, etc. has spread steadily, with the recent development of office automation.
The laser printer generally includes a pre-charger, a photosensitive body (e.g., drum and belt), an optical unit, a development device, a transfer device, and a fixing device. The pre-charger electrifies the photosensitive body uniformly (e.g., at xe2x88x92600V). The optical unit forms a latent image by exposure to light on the photosensitive body charged by the pre-charger. To be more specific, the optical unit irradiates a light from the light source on an area, and varies a potential on the irradiated area, for example, to xe2x88x9250V or so, to form an electrostatic latent image on the photosensitive body. The latent image is thereafter visualized into a toner image by the development device. The transfer device, which employs a corona charger for example, transfers the toner image onto the recordable medium or printing paper. The fixing device fixes the toner image that has been transferred on the printing paper, and the printing paper is then ejected.
The optical unit typically includes a light source section, a polygon mirror (rotating mirror), a lens system comprised of an f-xc3xa8 lens and a cylinder lens, a print start detector section (hereinafter referred to as BD sensor), and other necessary mirrors. These components are secured with a mounting part on a bottom surface of a housing, which is sometimes called optical box or optical base.
The light source section includes a light source, a collimating lens, and optionally a cylindrical lens. As the light source, for example, a semiconductor laser is used. The semiconductor laser irradiates a laser beam at a spreading angle from a point light source, so that the longer the radiating distance is, the wider the laser beam spreads out from the light source as a vertex of cone. The collimating lens, which is located near the light source, collimates the laser beam to make a parallel beam. The cylindrical lens subsequently forms the beam that has passed through the collimating lens into a beam converging in one direction. In order to achieve a large exposure area, high quality and fast writing action onto the photosensitive body, the number of semiconductor lasers to be provided is plural, and the equal number of collimating lenses, etc. is provided accordingly.
The polygon mirror, which is a deflector taking on a rotary polyhedron mirror, changes a laser beam traveling direction, and lets the laser beam scan. The lens system is provided to correct curvature, and to ensure constant velocity. The BD sensor detects a laser beam through a mirror, and provides timing for a print start.
In operation of the optical unit, the laser beam irradiated from the light source is collimated via the collimating lens. Thereafter, the laser beam is reflected by the rapidly rotating polygon mirror, and passes through the lens system, to correct curvature. The laser beam that has passed through the f-xc3xa8 lens passes through the cylinder lens while being detected by the BD sensor to provide print timing, scans the photosensitive body for a desired area to be exposed, and forms a latent image.
In order to securely form a high-quality latent image on the photosensitive body, the laser beam needs aligning with each optical axis of the collimating lens, the f-xc3xa8 lens, and the cylinder lens. A misalignment between the beam and each optical axis would lead to a shift of the beam in a sub-scanning direction. The shift of the beam in the sub-scanning direction would make the BD sensor unable to detect the beam, and result in varied pitches between the beams when the light source is made up of two beams. If the BD sensor cannot detect the beam, the print timing cannot be provided, and thus printing operations become impossible. Moreover, the varied pitches between the beams would, for example, thicken contours of the latent image, and deteriorate a print quality.
However, a conventional optical unit disadvantageously undergoes a thermal expansion by heat produced in the optical unit and other printer components during continuous operations of a light source, a lens system, or the like, and causes a misalignment between the beam and each optical axis. The heat is derived from heat sources such as a fixing device, a motor used for conveying a recordable medium, heat producing printed boards of various kinds, and a motor used for rotating a polygon mirror. Resultantly, temperature in the optical unit rises from room temperature (approximately 20xc2x0 C.) to approximately 60xc2x0 C., and the components in the optical unit thermally expand. Since the light source section and the lens system are different from each other in thermal expansion amounts due to differences in materials and shapes, the beam and each optical axis in the optical unit, which are properly aligned with each other in an initial state, would become misaligned as operations proceed. The present inventors and other colleagues, having assiduously considered a method of correcting the misalignment between the beam and each optical axis, have consequently put a focus on a method of mounting components in the conventional optical unit.
Referring now to FIG. 13, a description will be given more specifically of a structure of the conventional optical unit. FIG. 13 is a schematic perspective view showing main components in the optical unit 100B. As shown in the drawing, the optical unit 100B includes a light source section 10B, a polygon mirror 20B, an f-xc3xa8 lens 30B, and a cylinder lens 40B. The optical unit 100B is optically connected with a photosensitive drum 202. These components 10B, 30B, and 40B are respectively mounted on a base or housing 70B via mounting parts 102B through 106B. The optical unit 100B has a two-beam structure in which two light sources are provided.
A description will now be given of how the components 10B, 30B, and 40B are mounted with reference to FIGS. 14 through 16. FIG. 14 is a front view, side view, and exploded side view of the light source section 10B as viewed from a direction C in FIG. 13. FIG. 15 is a front view of the f-xc3xa8 lens 30B as viewed from a direction A in FIG. 13. FIG. 16 is a front view of the cylinder lens 40B as viewed from the direction A in FIG. 13. A dash-dot line in each figure represents the optical axes.
As shown in FIG. 14, the light source section 10B includes a semiconductor laser 12B as a light source, a collimating lens 11B that collimates a laser light irradiating at a spreading angle, a lens barrel 13B that holds the collimating lens 11B, and a block 15B that fastens the lens barrel 13B. A mounting surface fd between the light source section 10B and the mounting part 102B is located at a bottom portion of the light source section 10B, and positioned below the optical axis of the collimating lens 11B. The collimating lens 11B thermally expands from the mounting surface fd in a direction E (vertical direction, namely, sub-scanning direction), and thus the optical axis OA1 is shifted in the sub-scanning direction after the thermal expansion.
Similarly, referring to FIGS. 15 and 16, the f-xc3xa8 lens 30B and the cylinder lens 40B respectively include mounting surfaces fe and ff at each bottom portion. Therefore, when the lenses 30B and 40B thermally expand from the mounting surfaces fe and ff in a direction E, the respective optical axes OA2 and OA3 are shifted in the sub-scanning direction.
If the shift direction of the laser beam is represented with a main scanning direction and a sub-scanning direction, the main scanning direction corresponds to a circumferential direction of the polygon mirror, and the sub-scanning direction to a height direction thereof. Compared with a permissible shift amount in the main scanning direction, the permissible shift amount in the sub-scanning direction is much smaller. The laser beam, even if shifted in the main scanning direction, may be corrected for the deviation in the main scanning direction by adjusting a print timing with the rotating polygon mirror and the BD sensor. In contrast, the beam that has been shifted in the sub-scanning direction may fall on the polygon mirror, but possibly does not appropriately come in the mirrors and lenses that follow, and thus cannot be detected by the BD sensor. Consequently, print timing could not be provided properly, and printing function would be disabled. In addition, the optical unit in which two semiconductor lasers are provided would vary pitches between the beams, thicken contours of the latent image, and deteriorate a print quality. Therefore, the shift of the optical axis in the sub-scanning direction in the optical unit including a plurality of light sources is preferably smaller than that in the optical unit including one light source.
Referring now to FIGS. 18 and 19, a description will be given of a shift of an optical axis produced in a conventional one-beam type optical unit 100C. FIG. 18 is a schematic perspective view of the optical unit 100C. The optical unit 100C includes, like the embodiment illustrated in FIG. 13, a light source section 10C, a polygon mirror 20C, an f-xc3xa8 lens 30C, and a cylinder lens 40C. FIG. 19 is a partially enlarged perspective view of the optical unit 100C for explaining a shift of the optical axis of the f-xc3xa8 lens 30C that has thermally been expanded. As shown in FIG. 19, the lens 30C thermally expands from an interface surface (mounting surface fg) in a vertical direction (indicated by an arrow E, i.e., sub-scanning direction), and an optical axis position OA4xe2x80x2 after the thermal expansion is shifted from the original optical axis position OA4 in the direction E.
On the other hand, in order to avoid the misalignment due to the thermal expansion between the beam and the optical axes, it is also a conceivable idea to provide a cooler that cools down the inside of the optical unit, or an adjustment means for detecting and correcting the shift in the collimating lens in the sub-scanning direction, but this would unfavorably increase the device""s complexity and manufacturing cost.
Therefore, it is an exemplified general object of the present invention to provide a novel and useful optical unit, and electrophotographic recording device having the same, in which the above disadvantages are eliminated.
Another exemplified and more specific object of the present invention is to provide an optical unit that can align optical axes of lenses with a beam from a light source simply and inexpensively.
Still another exemplified object of the present invention is to provide an electrophotographic recording device that includes the above optical unit and thus can form a high quality latent image on a photosensitive body.
In order to achieve the above objects, an optical unit according to one exemplified embodiment of the present invention comprises a base, at least one lens that is disposed on a light path of a beam emitted from a light source, and a mounting part that mounts the lens on the base, and a mounting position between the mounting part and the lens is substantially aligned with an optical axis in a sub-scanning direction of the lens. According to this optical unit, since the lens may thermally expand more mainly above the mounting position, the optical axis on the mounting position, if low in thermal expansion coefficient, is shifted less from the original position.
An optical unit according to another exemplified embodiment of the present invention comprises a base, a collimating lens that collimates a beam from a light source; a scanning lens that corrects a curvature of the beam that has passed through the collimating lens, and a mounting part that mounts the collimating lens and the scanning lens on the base, and a mounting position between the mounting part and at least one of the collimating lens and the scanning lens is substantially aligned with an optical axis in a sub-scanning direction of the at least one lens. According to this optical unit, since the at least one lens may thermally expand more mainly above the mounting position, the optical axis on the mounting position, if low in thermal expansion coefficient, is also shifted less from the original position.
An optical unit according to still another exemplified embodiment of the present invention comprises a base, a collimating lens that collimates a beam from a light source; a scanning lens that corrects a curvature of the beam that has passed through the collimating lens, a first mounting part that mounts the collimating lens on the base, and a second mounting part that mounts the scanning lens on the base, and a first mounting position between the first mounting part and the collimating lens, and a second mounting position between the second mounting part and the scanning lens are disposed in a place substantially equidistant from the base. According to this optical unit, if the first and second mounting parts have the same thermal expansion coefficient, the optical axis of each lens is shifted less.
An electrophotographic recording device as one exemplified embodiment of the present invention comprises a photosensitive body, a pre-charger that electrifies the photosensitive body, the above optical unit that exposes the photosensitive body and forms a latent image thereon, a development device that visualizes the latent image into a toner image with a developing agent, a transfer device that transfers the toner image onto a recordable medium, and a fixing device that fixes the toner image on the recordable medium. This electrophotographic recording device has the same effect as the above optical unit.
In addition, an optical unit according to still another exemplified embodiment of the present invention comprises a base, at least one lens that is disposed on a light path of a beam emitted from a light source; and a mounting part that mounts the lens on the base, and a mounting position between the mounting part and the lens is substantially aligned with a position on the light path of the beam. Moreover, the above lens may have asymmetrical top and bottom lens surfaces. According to this optical unit, even if the lens has asymmetrical top and bottom lens surfaces, the same effect as the above optical unit may be produced.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.